Optimum conditions of body effect factor and substrate bias in variable threshold voltage MOSFETs

The effects of body effect factor (gamma) and substrate bias (V-bs) in a variable threshold voltage metal oxide semiconductor field effect transistor (VTMOS) have been systematically examined by device simulation. The characteristics of a VTMOS are significantly affected by the value of gamma and the V-bs difference (Delta V-bs) between the active mode and the standby mode. Optimal gamma and Delta V-bs to Obtain higher on-current in the active mode and lower off-current in the standby mode are derived. When off-current in the active mode is limited, a larger Delta V-bs and smaller gamma are preferable to obtain a higher drive current. When gamma is fixed, \Delta V-bs\ should be as large as the breakdown and leakage current permits. When Delta V-bs is fixed for some reason, such as breakdown, the optimum gamma depends on the relationship between Delta V-bs and V-dd: gamma should be larger when a large \Delta V-bs\ can be applied, while it should be smaller when the \Delta V-bs\ is small. The scalability of VTMOS is also discussed and it is found that channel engineering is strongly required in a scaled VTMOS. These results will greatly help in designing ultra-low power VTMOS VLSIs, and the VTMOS could be expected to survive in the 50 nm generation depending on the scaling scenario and applications.